This invention relates to geophysical exploration and more particularly to the filtering of multiple reflections from a seismic section.
A seismic section is a set of seismograms which depicts the subsurface layering of a section of the earth. It is the principal tool which the geophysicist studies to determine the nature of the earth's subsurface formations. Before an array of seismic samples can be converted into a seismic section which can be interpreted by the geophysicist, the seismograms must be processed to remove noise. One of the most frequently occuring types of noise is multiple reflections. These are caused by multiple "bounces" of the seismic energy between reflecting layers in the earth.
Various processes have been devised for suppressing multiple reflections. In one such process, multiple seismic coverage is obtained and stacked to suppress the multiple reflections. Various names have been given to the general process of obtaining multiple seismic coverage, e.g., common depth point techniques, common reflection point techniques, and roll-along techniques. All of these techniques involve the general principle of recording multiple seismic data from the same reflection point in the subsurface by employing variable horizontal spacing between a seismic source and seismic detector. These techniques are applicable to both marine and land seismic work. A description of such techniques is given by Lorenz Shock in an article entitled "Roll-Along and Drop-Along Seismic Techniques", published in GEOPHYSICS, Vol. XXVIII, No. 5, Part II, pp. 831-841, October, 1963. The data is corrected for normal moveout and statics and thereafter stacked.
Common depth point seismic techniques are generally credited with producing better seismic data than those techniques which produced singlefold seismic data. In stacking the common depth point seismic data, the primary reflections are essentially in phase and thus are added whereas the distortions such as multiple reflections are out of phase and tend to be cancelled. Thus the multiple reflections are suppressed and the primary reflections are enhanced.
Common depth point seismic techniques, in addition to yielding improved seismic data, also enable determinations to be made of velocity parameters of the earth. A knowledge of the velocity parameters of the earth from which seismic data is obtained is extremely important in the processing and interpretation of the seismic data. Various methods have been employed for obtaining velocity parameters from seismic data. One such method is described in U.S. Pat. No. 3,417,370 to Brey, which shows a typical system in which signal detection techniques are used to estimate acoustic velocity from seismic traces. Another method of obtaining velocity parameters is described in U.S. Pat. No. 3,651,451 to Ruehle. In accordance with the Ruehle patent, an index array of travel time curves specifying T versus X for a set of velocities passing through sample points on the outer trace of a common depth point set is computed. Seismic reflections in the traces are identified by detecting the signal across the traces along the travel time curves. In one particular signal detection technique, the traces are summed along each of the travel time curves in the set and the maximum signal power identifies the proper velocity.
Recently, workers in the field have transformed X-T seismic arrays representing amplitude of seismic reflections as a function of time and distance into f-k arrays representing amplitude as a function of frequency and wave number. My co-pending application, referred to above, and the prior art referred to therein, show the use of f-k transforms of seismograms. My co-pending application shows the filtering of the co-pending application, referred to above, and the prior art f-k transforms of seismic sections. The disclosure of that application is incorporated by reference herein.
While the foregoing, and other field procedures and processing techniques have been successfuly used, a need exists for improved multiple reflection suppression techniques.